The present invention relates to a structural body having a stochastic surface patterning, as well as a capacitive sensor having a structural body.
The measurement of dielectric constants, or even of the conductance, can be used for determining chemical substance properties. Thus, in cleaning processes, for example, the success of the cleaning can be inferred from the conductance of the washing liquid, or the composition of chemical compounds or mixtures such as of diesel fuels can be ascertained by measuring the dielectric constants.
In many cases, sensors for ascertaining these electrical properties of liquids and gases are composed of capacitor arrangements having interdigital comb structures, as are shown in FIG. 1. In this context, two electrodes are located opposite each other at a defined distance, while the substance to be analyzed is located in the interspace formed between them. In the practical case, the interspace is frequently implemented by slits having a constant spacing. The smaller the spacing of the slits, the more accurate the measurement becomes.
If the medium to be examined contains an impurity such as a coarse dust particle having a size in the order of magnitude of the interspace or greater, then this impurity will become caught on the electrodes, so that on one hand, the measuring result is invalidated, and on the other hand, the sensor is badly affected with time. It may be that the sensor can be cleaned by rinsing in the reverse direction, but many times jammed particles are not even removed with these means. In this respect, one must always expect a deterioration of the sensor. Finally, the sensor must either be disassembled for a rinsing, or the measuring system must have additional structural components for a rinse-solution pump.
Michael Kxc3x6hler, in xe2x80x9cxc3x84tzverfahren fxc3xcr die Mikroelektronikxe2x80x9d (Etching Methods for Microelectronics), 1998, WILEY-VCH Publishing House, particularly pages 322 ff., has already described methods for producing a stochastic distribution of differently formed pyramids on a surface of a material. From this, it is known in particular to produce a stochastic pyramidal surface patterning on a silicon wafer by a treatment with diluted KOH solution.
Compared to the related art, the structural body of the present invention having a stochastic surface patterning and the capacitive sensor of the present invention having such a structural body have the advantage that on one hand, because of the inhomogeneous electric flux lines resulting in response to the application of a voltage, a surface patterning of this type has a very high capacitance which is useful for exact measurements, and on the other hand, the surface patterning exhibits particularly good self-cleaning effects which are produced by the irregular spacings, arrangements and geometries of the multitude of geometric forms.
This self-cleaning effect is comparable to the cleaning effect of a fast-flowing water. Because of the irregular surface of the patterned areas, a medium flowing past does not exhibit a homogeneous flow distribution, e.g. within a channel, but rather fast-flowing regions and small swirls are formed. If a particle is now rinsed into the channel and is caught on a geometric form, the flow profile changes immediately and the particle is swirled around the geometric form. Thus, it xe2x80x9cby-passesxe2x80x9d this obstruction automatically.
Because of the randomly patterned area having a plurality of randomly or irregularly distributed geometric forms, enlargements always exist, both in the direction of flow and transverse thereto, through which corresponding swirls flow.
A surface patterning provided with a plurality of randomly or irregularly distributed geometric forms can also be used very advantageously in a capacitive sensor having a capacitor, the structural body or bodies being used in particular as capacitor plates which are in contact with a liquid or with a gas flowing past the surface.
Thus, it is very advantageous if the geometric forms in question have surface areas, heights or geometries varying as much as possible among each other and/or at least some of the geometric forms also partially overlap each other.
It is particularly expedient if, at least to a large extent, the geometric forms are pyramids. In this case, for example, electric flux lines run advantageously from pyramidal point to pyramidal point. Furthermore, the structural body is advantageously made of silicon, at least on the surface.
Moreover, the capacitive sensor of the present invention is very advantageously a plate-type capacitor whose capacitor plates are formed by two structural bodies according to the present invention which are each provided with a stochastic surface patterning along which a liquid or a gas flows. Such a sensor is very advantageously suited for analyzing the electric properties, in particular the dielectric constants or the conductance, of a multitude of different liquids or gases.
For that purpose, the sensor is advantageously provided, at least region-wise, with an electric contact, particularly a metallization of the structural body. Also advantageously provided is an electronic evaluation circuit which is connected to the capacitor and, in a manner known per se, ascertains a physical measurable quantity from which the dielectric constants and/or the conductance of the liquid or the gas, which in particular is flowing past, can be determined.